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Small wheel folding bike, which rc engine best for lightweight n performance ?

  1. Jan 18, 2010 #1
    I got a small 16inch folding bike weight 13kgs and managed to shed it down to 9kg and working in weight reduction further... eg using hollow shaft at the peddle and using clips at the axles and replacing to 14inch rims and redoing the frame in lighter stainless 201 - these are my next to-do lists.

    Riding in town is awesome experience and but riding between towns on small wheel bikes becomes tiring. I plan to cover say 40-50kms distance, or take public transport 1 way, and ride back.

    The RC motor should be lightweight and enough torque for flat runs,
    as power assist only, and if hills will stall the engine
    I am prepared to jump off the bike to push it.
    Friction mounting is easier to fit I think

    If I can get my small bike to say 6.5kg after completion of the to-do-list
    and add 1.5kg to the rc engine that would be perfect.

    Issues with RC engines
    eg for planes and cars - they rev fast eg 10000 rev/min
    small shaft
    no clutch
    capacity say 26cc or less...

    I am wondering which rc motor to use, where weight matter and still provide the
    performance and durability.

    Any input or feedback is greatly appreciated.
  2. jcsd
  3. Jan 18, 2010 #2
    I'm really fairly certain that you'd be better off rigging a brushless electric motor or something to your bike instead. The only way you're gonna be able to get an R/C engine to power a bike is with some very severe gear reduction. The smallest motor off hand that I would go with would be O.S. FS200 4 stroke, and even then it probably wouldn't have the torque to really do anything. With the slightest uphill slant it would probably stall.
  4. Jan 18, 2010 #3
    Some things that come to mind that might be of use:

    A lot of r/c car engines run on methanol/nitro/oil which is not cheap at around >$10US per litre.
    The r/c car I had 10 years ago was 0.12cu inch/2.04cm^3 capacity which supposedly would put out 0.5hp/~375W. It produced this power at around 30,000RPM. It had a centrifical clutch. I highly doubt you will find any r/c model engines that do not have a centrifical clutch.
    At the time, these 2 stroke engines required frequent retuning and could not be run for any longer than 15min or so as they needed to cool down (air cooled).

    A good cyclists puts out about 200W.

    You can buy whipper snippers that put out about 750W (~26cm^3) for ~$150US or so. These are noisy and smelly, but they are the same as what large scale (1:5th) r/c cars use.

    Check what your local legislation states about the power output your powered bicycle is allowed to have before you need to register it or comply with other legal requirements. In Australia, the legal limit is 200W. After that it is classed as a motor vehicle and starts to need to meet certain requirements and of course be registered.
  5. Jan 18, 2010 #4


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    R/C cars are capable of a lot more output these days, but you're right that fuel and maintenance would be prohibitive compared to a 2-stroke (or even 4-stroke) engine from a weed-whacker or chainsaw.

    A "big block" in R/C terms will be an engine in the .3-.5 cu. in. (4.9-6.5cc), and can easily put out power in the 2-3hp range (at around 30,000-40,000 rpm). So as was said it would take some pretty serious gear reduction to make one useful on a bicycle.

    Small 2-stroke (and now 4-stroke) engines in the 25-50cc range have become popular for powered bicycles, and aren't too expensive either. This is the route I would go for.
    Last edited: Jan 18, 2010
  6. Jan 18, 2010 #5


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    Here's a good motorized bicycle kit, and it costs significantly less than what a high-powered R/C engine would.

    http://www.bicycle-engines.com/images/images_big/Standard2Stroke.jpg [Broken]
    http://www.bicycle-engines.com/complete-48cc-powered-engine-p-1.html [Broken]
    Last edited by a moderator: May 4, 2017
  7. Jan 19, 2010 #6
    Thank you very much for all your responses.

    For the brushless motor suggestion,
    the advantage is in the simplicity in mounting the rc motor-alternator to produce the required amps to power the brushless motor .
    The 'generator' must be less than the weight of the battery pack. Note that the battery pack is dead weight and distance range is approx 20-30km, then its foot power for the remaining journey.
    Existing electric bikes weight approx 20kgs - 30kgs

    Bicycle-kit engines weighing in at approx 3-5kg
    and I have seen them used to cart gas cylinders / springwater totalling around 140kgs cargo weight and rider weight say 70kg - thats well over 200kgs and the engine kit could be 30cc or 50cc.
    The present bicycle-engine kit is simply too heavy and large
    to move a 70kg rider around, and I think the designers could make
    it much much smaller and lighter for recreational weekend casual cyclists.

    Weed Snipper, chainsaw engines friction mount looks lopsided, is still a weight issue, as the weighing in for these are 3kg +.

    Rotary engines have shorter drive shaft and narrow engine body,
    I am not sure if seals and fuel waste is still a disadvantage with the os engines - rotary wrankel models.
    The range of selection is limited also, they churn in well over 200W output power.

    If a pro cyclist leg power is aprox 200w, I think I will select a rc engine with similar range say 200-300w.

    I will look closely into the os fs 200 4stroke,
    if the drive shaft is too long compared to the rotary engines,
    then the mounting and friction assembly need more thoughts in the design, with weight considerations.

    I have seen the japanese chainless direct drive bicycles powering the 26inch wheel road bikes, the shaft and differential looks solid and heavy duty I am not sure what the weight is for this part, but i think it will be easier to attach the rc engine and turn it.

    So far, I am confident to shed more bike weight to 6kg or even 5.5kg, so as in the 1st post a 1.5kg for rc engine assembly -

    is this a tall ask or a possibility.

    Seems a simple wish and it has been fun to get from 13kg to 9kg.
    I can get to 5.5kg after the wishlist as stated in my 1st post.

    and motorised it ( add 1.5kg) to weigh in at 7kg.

    So the brainstorming goes on for me, and hey do feel free to pitch in...
    Again, thanks for all those who have responded.
  8. Jan 19, 2010 #7
    Keep in mind that the main obstacle to getting a bicycle to go faster is the overcoming of wind resistance which has a power requirement that goes up in proportion to velocity cubed.

    Something like 80% of power is required to overcome this over ~35km/h. Weight does not play a part in this. Weight is only an issue going up hills.

    I think the power gained from getting electrical assistance would more than make up for the added weight from the batteries. Especially if they are Lithium Polymer batteries.

    FYI: The best hill climbing cyclists in the world have a power (constant output over like an hour or so) of something like 6W/kg.
  9. Jan 19, 2010 #8
    Hahah, my friend has one of these kids on his bike. It really works!

    He painted the frame black and put a leather bike seat and black fenders on it. All he needs now are pink streamers and a bell on the handle bars.
    Last edited by a moderator: May 4, 2017
  10. Jan 21, 2010 #9
    What would you pick - a friction mount electrical motor or the back wheel electric hub type, and for the 2 choices what wattage is appropriate.

    When choosing electrical assistance, one need to plan the trip, ie how long
    will the lithium power pack lasts, and for pick-n-go travel trips the planning
    is necessary, ie one would take the gear along, but choose when to mount it.

    Oh yes, you mention in an earlier post re 200w for australia legals beyond that means rego required. Is the 200w measure when mounted on the bicycle or rack mounted engine. and how do they
    measure it -- for the amateur is there a way we can measure with simple garage tools?
    Last edited: Jan 21, 2010
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